The tautomeric forms of 4,6-dimethyl-2-mercaptopyrimidine in solution were examined. Experimental support for the existence of the tautomeric forms of this molecule was provided by infrared (FTIR), ultraviolet (UV), and nuclear magnetic resonance (NMR) techniques. Density functional theory (DFT) calculations allowed us to elucidate these molecular structures. The vibrational and electronic spectra, as well as the 1H and 13C chemical shifts, were satisfactorily described by DFT theoretical calculations. The potential of the DFT methodology was confirmed by prediction of a band at 2706 cm-1, associated with the stretching vibration of −SH group, which did not appear in the experimental spectrum because it was overshadowed by the symmetric and antisymmetric vibration modes of −CH3 groups. The existence of the tautomeric thiol−thione equilibrium of 4,6-dimethyl-2-mercaptopyrimidine with the thione structure dominating in polar solvents (CH3OH and DMSO-d 6) and the thiol structure in apolar media (cyclohexane) has been demonstrated.

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